See, guys? Women are just as frightened of us as we are of them. Perhaps this is the opportunity we've all been waiting for. Quick, marry them! Propose! PROPOOOOOSE!
In case my rousing call to action didn't nab you a few "panic engagements," you should know that the things that project confidence aren't incredibly gender-specific. Smiling, eye contact, a strong, clear voice, barrel of the gun held level and true. You know, the basics.
But since you call yourself a generally confident person, my guess is what you're really having trouble with is exhibiting those behaviors when the pressure is on. I know what that's like; no matter how comfortable I get acting on a stage, when I'm auditioning I always have to choose sitting monologues to hide my leg tremor.
So if you have a leg tremor, the answer is clear: learn to flirt while sitting. Otherwise try a few of my personal tricks for appearing confident.
* Pretend the other person has a terrible secret. Invent a hideous, hidden deformity or grave sin that makes them seem approachably pathetic.
* Build your confidence by starting small. Flirt with your pets or household appliances (countertop to begin with, then move up to washer/dryer units). Once you've successfully dated and crushed the hearts of the collected objects in your house, you can start in on people uglier than you, and so on until your confidence with guys is at a normal level (ie, you expect them to buy you things for the privilege of breathing your air).
* Remind yourself that we are all of us just human beings, with the same fears, the same wants, the same wisdoms and ignorances. Then when Lady Gaga's bodygaurds start dragging you off, kick your shoes at her and promise to "see her soon."
* Keep a flirt diary. Treat it like a skill you want to hone or a game you're practicing. Held eye contact is one point, a three-minute conversation is two points, and so on. Keep trying to beat your own score until you're married to everyone.
* I gain a lot of confidence by pretending to be another person. Costumes, an internet persona, anything that separates "the real me" from the people I'm interacting with. Since donning full Victorian attire might not be conducive to flirting, try just wearing a pair of dark sunglasses, and imagining yourself as a double agent infiltrating a ring of cute frat guys/international terrorist cell.
If you can pull the actual advice out of that list, I'm pretty sure there's something that makes some kind of sense. If not, I apologize, but I'm not re-doing it. Good luck!